1. Field of the Invention
This invention relates to polycrystalline diamond cutters mounted to insert studs that are mounted within the body of a rotary drag bit.
More particularly, this invention relates to polycrystalline diamond cutting elements that are formed in a convex shape and mounted to tungsten carbide studs that are subsequently secured within insert holes formed within the cutting face of a rotary drag bit.
2. Description of the Prior Art
Flat diamond cutting disks or elements mounted to tungsten carbide substrates are well-known in the prior art. Insert blanks or studs, for example, are fabricated from a tunsten carbide substrate with a diamond layer sintered to a face of the substrate, the diamond layer being composed of a polycrystalline material. The synthetic polycrystalline diamond layer is manufactured by the "Specialty Material Department of General Electric Company of Worthington, Ohio". The foregoing drill cutter blank goes by the trademark name "Statapax Drill Blanks". The Stratapax cutters, typically, are comprised of a flat thin diamond disk that is mounted to a cylindrical substrate which in turn is brazed to a tungsten carbide stud. Typically, the Stratapax blanks are strategically secured within the face of a rotary drag bit such that the cutting elements cover the bottom of a borehole to more efficiently cut the borehole bottom thereby advancing the drag bit in a borehole.
Drag bits with strategically placed Stratapax type inserts in the face of the bit also require a generous supply of coolant liquid to cool and clean the Stratapax cutters as they work in a borehole. It is well-known in the drag bit art that if diamond material is exposed for a prolonged time in a borehole without adequate cooling, the overheated diamond will convert to graphite.
Since the polycrystalline diamond disk of the Stratapax cutter is flat, the detritus or debris from the borehole bottom tends to pile up against the face of the diamond cutter thereby inhibiting a flow of coolant past the cutting face of the cutter thereby interfering with the cooling effect of the liquid against the cutting face of each of the diamond cutters.
U.S. Pat. No. 4,570,726 describes cutter elements for drag-type rotary drill bits which consists of forming an abrasive face contact portion into a curved shape. The curved shape directs the loosened material to the side of the contact portion of the abrasive element. The curve however, is in one plane so that the rake angle, with respect to a centerline of a drag bit, is constant thereby providing a stagnation point along this plane which would tend to ball or jam the cutter as it works in a borehole.
Principles of heat transfer and fluid dynamics teach that the convection heat transfer coefficient for a body, such as a cutting element for a drag bit, passing through a fluid varies greatly depending on the shape of the body. Planar faces having fluid flowing normal to them are among the least effective at convective cooling in the fluid. This result is caused in part by the stagnation layer in the fluid that is set up against the working surface of the cutting element. Since the insert, as taught by this invention, has a constant planar surface or rake angle, the cooling effect of the fluid along this plane would be somewhat minimized.
The polycrystalline cutting element of the present invention is spherically shaped, rather than just a curved planar surface. The rake angle, whether it is in a substantially vertical plane or a horizontal plane is constantly variable, thus the convex cutting element moves through a liquid medium with the greatest possible transfer of heat from the diamond cutting face to the fluid. The spherical cutting element of the present invention would have a definite advantage over the foregoing invention.
U.S. Pat. No. 4,593,777 describes a stud type cutting element having a diamond cutting face, the cutting face being adapted to engage an earth formation and cut the earth formation to a desired three-dimensional profile. The cutting faces defined a concave planar surface in one embodiment which has back rake angles which decrease from the distance from the profile. While the rake angle changes with penetration of the insert in a formation it changes in only the vertical plane, the horizontal plane remains constant, thus detritus would tend to pile up in front of this concave planar surface. Another embodiment discloses an insert having a circular concave surface with a negative rake angle with respect to a formation bottom. This type of insert would direct the detritus towards the center of the cutting element, thus balling the face of the cutting element, thereby detracting from the efficiency of the cutter and adding to its destruction by preventing adequate cooling of fluid to the cutting face.
The present invention teaches the use of a convex or spherical diamond cutting surface that has infinitely changing rake angles, both in the vertical and the horizontal plane. The curved surfaces provide maximum cutting capability and maximum cooling efficiencies since detritus
is moved away from the center of the inserts in all planes. The rake angle is constantly variable as the penetration varies during operation of the drag bit in a borehole.